The present invention relates to a method for the preparation of guanidine sulfamate or, more particularly, to a method for the preparation of guanidine sulfamate by the reaction of an ammonium aqua-ammonosulfate and urea.
It is known that the reaction of urea and an ammonium aqua-ammonosulfate such as ammonium sulfamate, ammonium imidosulfonate, ammonium nitridosulfate and the like gives a reaction mixture mainly composed of guanidinium ions, ammonium ions, sulfamate ions, imidosulfonate ions and sulfate ions. This reaction may be carried out either in air or in an atmosphere of ammonia and also under atmospheric pressure or under a pressurized condition. The reaction mixture obtained by the reaction under atmospheric pressure is characterized by the higher content of the imidosulfonate ions than the reaction mixture obtained by the reaction under pressurization with ammonia.
It is an industrially feasible process to obtain guanidine salts such as guanidine sulfate and guanidine sulfamate useful, for example, as a flame retardant from the above reaction mixture by undertaking a suitable means for separation. Unfortunately, the separation of the guanidine salts from the reaction mixture is not a simple matter because so many kinds of ionic species are coexistent in the reaction mixture and most of the coexistent salts have a very large solubility in water. For example, no satisfactory results are obtained by a conventional physical or physicochemical method for separation such as precipitation of the salts by chilling or evaporating the solution or by the addition of a water-miscible organic solvent to the aqueous solution to decrease the solubility of the salt and a method by use of an ion-exchange resin. Accordingly, several methods for the separation have been proposed including a chemical process but none of them is satisfactory with its own problems from the standpoint of practicability.
For example, the inventor has previously disclosed a method for the separation of the guanidine salts in which the reaction mixture of urea and an ammonium aqua-ammonosulfate is treated with calcium carbonate to separate the desired guanidine sulfamate (see Japanese Patent Publication 50-21452). This method is, however, very time-consuming taking an extremely prolonged reaction time for completing the reaction with calcium carbonate by boiling an aqueous solution of the reaction mixture. Therefore, this method is economically disadvantageous, especially, when the cost for heating energy is large. In addition, this method of treatment with calcium carbonate is not applicable to the reaction mixture obtained by the reaction under atmospheric pressure with ammonium imidosulfonate or ammonium nitridosulfate as the ammonium aqua-ammonosulfate because of the presence of ammonium imidosulfonate in the reaction mixture.
With an object to overcome the above described difficulties, the inventor has further proposed an improved method (see Japanese Patent Disclosure 50-96527) for separating guanidine sulfate from the reaction mixture in which the reaction mixture obtained by the reaction under atmospheric pressure is heated in water at a temperature of 100.degree. C. or higher to be converted into a mixture composed of guanidinium ions, ammonium ions, hydrogen ions and sulfate ions which is then treated with calcium oxide or calcium hydroxide. This method, however, must be carried out under a pressurized condition giving a great limitation to the practical application of the method.